President Barack Obama gave an ultimatum to Libyan leader Moammar Gadhafi on Friday: Cease all violence against your people or prepare for United Nations-backed military action.

The U.S. “won’t stand idly by in the face of actions that undermine global peace and security,” Obama said during somber remarks at the White House. “Left unchecked, we have every reason to believe Gadhafi would commit atrocities against his own people” and the “entire region could be destabilized.”

The president said Gadhafi has two choices: Pull back your forces from several cities and restore basic amenities to your people, or face military action from the international community. “Let me be clear: these terms are not negotiable,” he said. “If Gadhafi does not comply ... the international community will apply the consequences.”

Obama said he has directed Defense Secretary Robert Gates and Secretary of State Hillary Rodham Clinton to begin working immediately with international allies to enforce a no-fly zone over Libya. Clinton will leave for Paris on Saturday to help roll out United Nations Security Council Resolution 1973, which bans all flights on Libyan airspace, save those with a humanitarian purpose.

Obama said he has “no doubt” that the U.S. military is capable of carrying out this mission but emphasized what the United States will not be doing: sending ground troops into Libya.

“The United States is not going to deploy ground troops into Libya. We are not going to use force to go beyond a well-defined goal,” he said.

The president said while it was a difficult decision to use military force in Libya, particularly given the strain of the wars in Afghanistan and Iraq, the United States is part of a broader international effort to bring Gadhafi down. France, Great Britain and members of the Arab League are taking the lead in enforcing the U.N. resolution.

“We are coordinating closely with them. This is precisely how the international community should work,” he said.

Reactions on Capitol Hill were far and wide.

Sen. Chuck Grassley wondered aloud whether Obama’s move is too little too late — and whether the U.S. should go further. “Mayb we won’t lose Libya now that obama has decided to take action Bettr to do 3wks ago. Can’t rely on UN,” read a tweet from the Iowa Republican.

Meanwhile, Rep. Jerrold Nadler, the ranking member on the Judiciary Subcommittee on the Constitution, reminded Obama that he needs Congressional approval before he can take any military action.

“The president has an obligation under the Constitution to seek the approval of Congress for any use of military force unless there is an imminent threat to the United States or its allies,” the New York Democrat said.